[Cast] In Audio/VideoSender, drop frames when too-long a duration is in-flight.

media/cast/sender/rtp_timestamp_helper.h

Index: media/cast/sender/rtp_timestamp_helper.h
diff --git a/media/cast/sender/rtp_timestamp_helper.h b/media/cast/sender/rtp_timestamp_helper.h
index 8f56681dac3651579884f8ce619a61718677c21a..28fe3a1e9801be442ddc1ca8b0b544a1f6fcfef4 100644
--- a/media/cast/sender/rtp_timestamp_helper.h
+++ b/media/cast/sender/rtp_timestamp_helper.h
@@ -17,11 +17,18 @@ class RtpTimestampHelper {
   explicit RtpTimestampHelper(int frequency);
   ~RtpTimestampHelper();
 
-  // Compute a RTP timestamp using current time, last encoded time and
-  // last encoded RTP timestamp.
-  // Return true if |rtp_timestamp| is computed.
-  bool GetCurrentTimeAsRtpTimestamp(const base::TimeTicks& now,
-                                    uint32* rtp_timestamp) const;
+  // Compute a RTP timestamp from a TimeTicks, or a TimeTicks from a RTP
+  // timestamp; based on the currently stored capture time and its corresponding
+  // RTP timestamp.  Return true if |rtp_timestamp| is computed.
+  //
+  // The values are estimates because, ultimately, it's the encoders that decide
+  // what the RTP timestamps for each frame are going to be.  For example, in
+  // the case of audio, the TimeTicks may never be precise to an exact frame
+  // boundary; and so the computation here will result in a |rtp_timestamp|
+  // value that is near, but never exact to one emitted by the encoder.
+  bool EstimateRtpTimestamp(const base::TimeTicks& t,
+                            uint32* rtp_timestamp) const;
+  bool EstimateTimeTicks(uint32 rtp_timestamp, base::TimeTicks* t) const;
 
   // Store the capture time and the corresponding RTP timestamp for the
   // last encoded frame.